Print heads employed in inkjet printers and the like usually each contain a plurality of nozzles arranged in one or more linear arrays parallel to the propagation direction of the image-receiving member or in other words the sub scanning direction. The nozzles usually are placed substantially equidistant. The distance between two contiguous nozzles defines the nozzle pitch.
In operation, the nozzles are controlled to image-wise discharge ink droplets on an image-receiving member so as to form columns of image dots of ink in relation to the linear arrays such that the printing pitch equals the nozzle pitch. In scanning inkjet printers, a matrix of image dots of ink, corresponding to a part of an image, is subsequently formed by scanning the print heads across the image-receiving member, i.e. in the direction perpendicular to the propagation direction of the image-receiving member or in other words the main scanning direction. After a first matrix is completed, the image-receiving member is displaced so as to enable the forming of the next matrix. This process may be repeated till the complete image is formed.
An advantage of forming an image of image dots of ink on an image-receiving member as here described is the high productivity using only a single printing stage. However, image quality may be improved by employing printing devices enabling the use of multiple printing stages. Conventionally, two main categories of such printing devices can be distinguished into so-called “interlace systems” and “multi-pass systems”.
In an interlace system, as e.g. disclosed in U.S. Pat. No. 4,198,642, the print head contains N nozzles, which are arranged in one ore more linear arrays such that the nozzle pitch is an integer multiple of the printing pitch. Multiple printing stages, or so-called interlacing printing steps, are required to generate a complete image. According to the disclosure in the above-mentioned patent, the print head and the image-receiving member are controlled such that in ‘I’ printing steps, ‘I’ being defined here as the nozzle pitch divided by the printing pitch, a complete image part is formed on the image-receiving member. After each printing step, the image-receiving member is displaced over a distance of N times the printing pitch. Such a system is of particular interest because it allows to achieve a higher print resolution with a limited nozzle resolution.
In a multi-pass system, the print head contains N nozzles, which are arranged in one or more linear arrays. In operation, the print head is controlled such that only the nozzles corresponding to selected pixels of the image to be reproduced are image-wise activated. As a result an incomplete matrix of image dots is formed in a single printing stage, i.e. a horizontal scanning pass across the image-receiving member in one direction. Thus, multiple passes are required to complete the matrix of image dots. In-between two passes, the image-receiving member may be displaced in the sub scanning direction.
Both the “interlace systems” and “multi-pass systems” as well as combinations thereof share the advantage of an improved image quality and the inherent disadvantage of a lower productivity. Such systems are known to be of particular interest to overcome or at least reduce the visibility of some banding artifacts, particularly regional banding artifacts. Regional banding artifacts are caused by irregularities which can be attributed to individual nozzles or small regional clusters of nozzles within the array(s). Such irregularities may lead to regional variations in dot-size or dot positioning. Examples of such irregularities are differences in nozzle shape or size, differences in the shape or size of the ducts connecting the ink reservoirs with the respective nozzles. These differences can occur in the manufacturing or may arise during use, e.g. caused by contamination of the ink.
The so-called print mask contains the information about the number and sequence of printing stages and defines which nozzles need to be activated. In other words, the print mask contains the information defining for each printing stage which pixels will be rendered by which nozzles such that when all printing stages are completed, all the pixels are rendered. Conventional print masks are usually configured so as to minimize the influence of random regional variations in dot size and positioning. A print mask is associated with a printing mode. Selecting a printing mode enables the user to exchange image quality for productivity and vice versa dependent on the user's requirements. By selecting a printing mode also the nozzles on the print head which will be effectively used are determined as well as the displacement step in the sub scanning direction after each printing stage.
However besides banding artifacts caused by the abovedescribed regional variations in the dot-size or positioning, also very disturbing banding artifacts caused by so-called systematic variations in the dot-size can arise in “interlace systems” and “multi-pass systems” as well as combinations thereof. Systematic dot-size variations are caused by differences in the size of dots formed by different groups of nozzles. For instance, in a print head comprising two linear arrays of nozzles for the same colour, the first group of nozzles may constitute the first array of nozzles while the second group of nozzles constitutes the second array of nozzles. When due to a small shift in the manufacturing process all nozzles of the first array are sized slightly different from the nozzles of the second array, systematic variations in the dot-size can arise between droplets originating from the nozzles of the first and second group. Another example is a print head comprising a single linear array of nozzles for a particular colour wherein the nozzles are controlled such that first the even nozzles within the array, i.e. the first group of nozzles, are discharged and thereafter the odd nozzles within the array are discharged. Again this may lead to a systematic dot-size variation which in case of a thermal or thermal-assisted inkjet printer may be caused by e.g. a small temperature variation, or in case of a piezoelectrical inkjet printer may be caused by e.g. mechanically induced cross-talk. A further example is an ink-jet printer comprising multiple print heads for a particular colour wherein the respective groups are constituted by the respective arrays of the respective print heads. In such a configuration, again e.g. small differences in the nozzle sizes of nozzles groups each associated with a different print head may lead to systematic dot-size variations.